This invention relates to a microwave heating apparatus.
In the microwave heating apparatus, a microwave generated from microwave energy oscillator is applied through a waveguide to an object to be heated and the object is subjected to dielectric heating by the microwave.
When microwave energy heating is executed, the degree of absorption of the microwave in an object to be heated varies with time, such as with the reduction in the quantity of the object. Therefore, the load impedance varies and the voltage standing-wave ratio (VSWR value) in the waveguide changes, and consequently it becomes impossible to cause microwave energy from the waveguide to be absorbed efficiently in the object.
In order to prevent the lowering of efficiency due to the change in the VSWR value, it has been a usual practice that an operator adjusts manually a stub for adjustment provided in the waveguide, while watching the data denoting the incident and reflected power of the microwave energy in the waveguide, so as to vary the waveguide impedance appropriately for impedance matching and thereby to maintain the VSWR value within a predetermined range where favorable matching achieved.
However, such manual adjustment as stated above suffers the drawback of being troublesome and taking much time and labor, when the microwave heating continues for an especially long time or when the degree of absorption of the microwave energy in an object to be heated varies each heating.
To cope with this problem, there has been proposed a method wherein several kinds of expected variation patterns of the VSWR value are set beforehand on the basis of experience and properly selected in accordance with a change in the load impedance of the object, so as to vary the waveguide impedance and thereby to adjust the VSWR value.
By this method, however, the impedance matching can not be performed well and lowering of the heating efficiency is unavoidable when the VSWR value suddenly varies in an unexpected pattern other than the patterns prepared beforehand or when the selected variation pattern does not coincide with the actual change in the load impedance of the object to be heated.